1. Field of the Invention
The present invention relates to a wind turbine and a method of controlling such a wind turbine, in particular a method of controlling a wind turbine to reduce maximum loads experienced by the wind turbine during extreme wind conditions.
2. Description of Related Art
Wind turbines can often be located in areas having relatively predictable wind patterns, e.g., varying between ˜15-25 m/s. However, during storm conditions wind speeds can often reach extreme levels capable of damaging wind turbine structures. For example, off-shore wind turbine installations may experience typhoon or hurricane conditions, wherein the wind speed may exceed 70 m/s during gusts. The high wind speeds mean that wind turbines intended for a site susceptible to extreme wind conditions have to be constructed with sturdier materials and/or additional reinforcement elements, in order to withstand the effects of the high winds possible in such areas, and to be rated suitable for use in the locations in question. Furthermore, high wind speeds during gusts can result in significant fatigue loads in the structural components of the wind turbine, which can lead to additional wear-and-tear on the wind turbine structure. Accordingly, it is of interest to find ways to reduce the impact of extreme wind conditions on wind turbines.
European Patent Application Publication No. 0 709 571 describes a two-bladed partial pitch wind turbine which reduces the effect of extreme wind conditions. The turbine comprises first and second rotor blades, having inner and outer blade sections, the outer blade sections pitchable relative to the inner blade sections. During high winds, the rotor blades are parked in a substantially horizontal alignment, and the outer section of the first blade is pitched to be at a 90 degree angle to the inner section of the first blade, while the outer section of the second blade is unpitched. The azimuth or yaw brake is released, and the rotor structure comprising the first and second rotor blades acts as a wind vane when exposed to high winds. As a result, the rotor is moved about the yaw axis such that the tip end of the first rotor blade is pointing directly into the oncoming wind, and consequently presents a reduced surface area against which the wind acts on. The reduced surface area results in reduced forces on the turbine during the high wind conditions, and reduced loading in the wind turbine structure.
Several problems exist for this solution however. For example, in the case of relatively small wind turbines or precisely-balanced wind turbines, there may be excessive wear-and-tear in the yawing system, as the rotor blades may be continually in rotational motion due to minor variations in the wind direction at the turbine. Such constant rotation can wear on the geared wheels, etc., present in wind turbine yawing systems, necessitating early repair and/or replacement.
Alternatively, in the case of relatively large wind turbines, i.e., turbines wherein the moment of inertia for yawing is relatively large, the wind vane (i.e., the rotor blades) will not be turned until the wind exceeds a wind speed sufficient to overcome the yawing inertia of the rotor blades. Accordingly, during an extreme wind condition, the wind may enter a period of calm for a period of time, during which the wind speed falls below the level required to yaw the rotor blades. If the wind direction changes during this period, and if the wind levels subsequently rise sharply, e.g., in the form of a gust, the full force of the extreme wind may impact on a majority of the surface area of the rotor blades, resulting in maximum loads experienced by the wind turbine structure.
Also, U.S. Pat. No. 8,100,628 B2 describes a solution where a rotor of a wind turbine is positioned in a rest position during a malfunction of the blade adjustment equipment. The rest position is preferably 90 degrees to the wind direction and the position of the nacelle can be adjusted according to a change in the wind direction, by activating the azimuth drive of the wind turbine. There is however not disclosed any method for deciding when to adjust the position of the nacelle, but performing a full correction of the position of the nacelle for every small change in wind direction will lead to excessive wear-and-tear in the yaw system as mentioned above.